1. Field of the Invention
The invention generally relates to a digital broadcasting system in which a plurality of channels are transmitted as a time-division multiplexed data stream, each channel comprising a plurality of continuous or time-series multimedia programs. It relates more particularly to a method of and a system for downloading a segment or the whole of a received continuous program from the data stream with a raised preciseness in such a digital broadcasting system.
2. Description of the Prior Art
Overview of a Conventional Multiplexing Technique
FIG. 1 is a schematic block diagram showing an exemplary arrangement of a conventional digital broadcasting system 1 to which the present invention is applicable. In FIG. 1, the broadcasting system 1 comprises at least one broadcasting station 2, a transmission medium 3 and a multiplicity of receiving terminals 4.
In a multimedia digital broadcasting system, a video, one or more audio and various data constitute a program. A plurality of such programs for respective channels is multiplexed into a data stream. Coding and multiplexing in most cases is achieved according to a international standard for high efficiency coding and multiplexing, known as MPEG-2 (Motion Picture Experts Group II) (ISO/IEC 13818). For this, the following description will be given in connection with the MPEG-2 standard.
In an MPEG encoder unit 201 of the broadcasting station 2, a video is coded into a video MPEG-coded bit stream, which is then packetised into video PES (packetised elementary stream) packets (according to ISO/IEC 13818-2). An audio is separately coded into an audio MPEG-coded bit stream, which is packetised into audio PES packets (according to ISO/IEC 13818-3). FIG. 2 is a diagram showing a structure of a PES packet 900, wherein a sectioned strip bounded by broken lines fanning out downward form a bold-lined section shows a detailed structure of the bold-lined section and wherein a numeral under each field indicates the length of the field in bits. The PES packets 900 are variable length packets, which are used to synchronize the coded bit streams for a program. Each PES packet 900 comprises a header 901 and PES packet data bytes 902.
The video and audio PES's (or PES packet streams) that constitute a program are multiplexed into a transport stream (TS), which comprises TS packets shown in FIG. 3. FIG. 3 is a diagram showing a structure of a TS packet 910, wherein a sectioned strip bounded by broken lines fanning out downward form a bold-lined section shows a detailed structure of the bold-lined section. In FIG. 3, a TS packet 910 is fixed, i.e., 188 bytes in length and comprises a header 911 and a payload 912. The header 911 contains information on the contents of the payload 912, including a 13-bit packet ID (PID) 913 for identifying the contents of the payload 912.
The multiplexing of the PES packet streams is achieved by dividing and storing each of the PES packets 900 into and in one payload 912 after another of the TS under creation. In this case, the values of the PIDs 913 in the headers 911 of the TS packets 910 are so set that the value of each PID 913 is associated with the PES packet stream a part of which is contained in the corresponding payload 912. Thus, continuous (or time-series) media materials that constitute a multimedia program are multiplexed into a TS. A plurality of such TSs corresponding to respective channels is further multiplexed into another TS. The operations described so far are executed in the MPEG encoder unit 201. In order to discriminate the first created TSs from the finally created TS, the former and the latter are referred to as "logical channel TS (LC TS)" and "physical channel TS (PC TS)", respectively.
The physical channel TS is transmitted by a transmitter 202 into a transmission medium 3.
On the other hand, a receiving terminal 4 comprises a tuner 210 for receiving a plurality of physical channel TSs and providing a selected physical channel TS, which has been multiplexed in accordance with the MPEG-2 standard; a TS decoder 211 for providing a selected logical channel TS from the received PC TS; a PES decoder 212 for extracting the PES packets 900 from the payloads 912 of the TS packets from the TS decoder 211 and demultiplexing the PES packets 900 into video and audio PES packet streams according to the PIDs 914 in the TS packet headers 911; and a presentation decoder 213 for restoring video and audio bit streams from the video and audio PES packet streams by MPEG decoding the video and audio PES packet streams separately. The tuner 210, the TS decoder 211, the PES decoder 212 and the presentation decoder 213 may be any suitable conventional ones. However, it is noted that the MPEG decoding has to be achieved such that the decoded video and audio bit streams synchronize with each other.
For this purpose, the digital broadcasting system 1 is arranged as follows.
Synchronization Technique
The broadcasting station 2 has a system time clock (STC) generator 203. The generated STC is a 42-bit number n that is incremented at a frequency of 27 MHz. The STC n is contained in a program clock reference (PCR) field 914 in the header 911 of each TS packet 910. Also, the broadcasting station 2 is permitted to store a presentation time stamp (PTS) and/or a decode time stamp (DTS) in a PTS 903 and DTS 904 fields in a optional header portion 905 of a PES packet 900 if the PES packet 900 contains the head of an access unit of an MPEG-coded bit steam in its packet data bytes field 902 (An access unit is one frame in case of video bit stream and one audio frame in case of audio bit stream). The PTS and DTS are represented by 33 bits with a precision of 90 KHz. Thus, the receiving terminal 4 can synchronize a plurality of continuous media materials with each other by decoding and presenting the continuous media materials such that the PTSs of the media materials coincide with a corresponding regenerated STC from a STC regenerator 214. The STC regenerator 214 is a PLL(phase locked loop)-based circuit that provides a regenerated 42-bit STC value n at a frequency of 27 MHz according to the values of the PCR fields 914 of the headers 911 of the TS packets 910 supplied from the TS decoder 211 while keeping the error with respect to the PCR 914 value within a certain range. It is noted that the TS decoder 211 is configured to ensure a high precision of the delay time from input of a TS packet 910 from the tuner 210 to extraction of an STC from the PCR field 914 of the TS packet 910.
However, since the STC is a clock specific to the broadcasting system 1 and different from ordinary time we use in our daily life, the STC is inconvenient for us to use in operating and programming the receiving terminal 4. For this, a clock that provides ordinary time is required.
As such a clock, EIT (Event Information Table) and TDT (Time and Date Table) are available which are prescribed in a DVB-SI (Digital video broadcasting--Service Information) standard (ETS 300 468) established by a standardization organization ETS (European Telecommunication Standard). The EIT contains the start and the duration of each event or program. The TDT is a time in which year (y), month (mo), date (d), hour (h), minute (m) and second (s) are expressed in a form known as UTC (Universal Time Co-ordinated) form. The TDT is used for reference to an event or program. (The time according to the TDT is referred to as "reference time"). In Japan, Japanese Standard Time is used as the reference time. The reference time is used in, e.g., displaying a program guide according to EPG (electronic program guide) and programming a VTR (video tape recorder). The broadcasting station 2 preferably has a TDT receiver 204 for receiving the TDT data. The broadcasting station 2 transmits TDT data in the well-known section format.
In order to enable a conversion between a regenerated STC and a corresponding reference time, the broadcasting station 2 also transmits a reference STC value (denoted by N0), which is again expressed by 42 bits and variable by a step of 1/27MHz. A value of STC at 0:00 am in reference time is preferably used as the reference STC value. The reference STC value is transmitted in a format known as "section" defined in the above-mentioned DVB-SI standard. The reference STC value may be divided into subtables called sections in transmission. It is noted that the section format is intended for repeated transmission of same information and is not guaranteed for synchronization or constant delay.
Conventional Downloading Techniques
FIG. 4 is a diagram showing one conventional technique for clipping a desired portion of a received TS. It is assumed that a desired portion of the received TS is specified by the start time Ts and the end time Te of the desired portion (Ts and Te is expressed in the above-mentioned reference time) and that the start and end times are given by a user directly specifying them or by the broadcasting station transmitting event information (e.g., EPG, EIT, etc.) including the channel ID, the segment ID, Ts and Te of a time segment of a channel and the user selecting a desired time segment. In this technique, The received TS is clipped at the given start and end times Ts and Te measured by a local timer provided in a receiving terminal.
However, the conventional technique provides only a lower clipping precision due to a significant error between the scheduled time and the actually transmitted time which error is affected by buffering in the transmitter 202 and the receiving terminal 4, a transmission delay in the transmission media 3, and the error between the timers of the broadcasting station 2 and the receiving terminals 4.
FIG. 5 is a flowchart showing an operation executed by the controller 215 in another downloading technique. In FIG. 5, the start and end reference times Ts and Te is obtained in a manner described in the above technique in step 251. In order to enable the clip range to be specified with a higher precision, it is assumed that each of the times Ts and Te can be expressed in a combination of a reference time (whose unit is a second) and the number f of frames (0.ltoreq.f.ltoreq.29). Specifically, let a reference time t (e.g., Ts or Te) be expressed in the form of "y:mo:d:h:m:s:f". In step 252, the start and end reference times Ts and Te are converted into 42-bit STC values Ns and Ne by using the above-mentioned 42-bit reference STC value NO in a manner like: EQU n=[N0+(27.times.10.sup.6 /F).times.{(60.times.(60h+m)+s).times.F+f}]mod(2.sup.42),
where n is an STC value which corresponds to a reference time t, F is the number of frames per second, and XmodY is the residue of X/Y. In step 253, the STC values Ns and Ne is further converted into 33-bit STC values Ns' and Ne' so as to be compared with presentation time stamps (PTSs) which are also 33 bits in length. The method of this conversion is detailed in the MPEG-2 standard. FIG. 6 is a diagram showing how a clipping is started for PES packets P0, P1, . . . , wherein a small rectangle at the head of each packet Pi indicates the header 901 thereof, and blacked ones among the small rectangles indicate headers 901 with values in their PTS fields 903. In step 254, a test is made to see if the PTS 903 value of the current PES packet Pi (i=0, 1, . . . ) is equal to or larger than the 33-bit start STC value Ns'. The test of step 254 is repeated till the test result becomes YES. If the test result is YES in step 254, clipping is started with this PES packet Pi (a packet P6 in FIG. 6 for example). In step 256, another test is made to see if the PTS 903 value of the current PES packet Pj (j is an integer larger than i) is equal to or larger than the 33-bit end STC value Ne'. The test of step 256 is repeated till the test result becomes YES. If the test result is YES in step 256, clipping is stopped immediately. The operation of steps 254 through 257 is executed for both of the video and audio PES packet streams. In this way, a desired portion is clipped from the received continuous media.
However, this technique has to execute, in real time, the steps 254 through 257 which involve a comparison with the value of the PTS field of each PES packet, which may require hardware dedicated to such steps. If such hardware is not provided, the controller 215 has to monitor every PTS field even when either end of a clipped portion is not supposed to appear, causing the total load of the terminal 4 to increase. This results in an increase in the power consumption during the stand-by.
It is therefore an object of the invention to provide a method of downloading a desired portion of a received continuous medium with a raised precision without increasing the load in processing, and to provide a broadcasting system capable of such downloading.